The utility of charge-derivatized peptides has been a substantial development from this Facility, first in the context of FAB analyses, and more recently in the context of MALDI. It was unexpected that charge-localized peptides respond well in MALDI. The mechanism of ionization in this experiment is not well-understood, but is presumably similar to the mechanism through which other cations such as sodium ions are formed. As in the case of protonated peptides, charge-derivatized peptides form only a single peak in a MALDI mass spectrum using a linear time-of-flight mass spectrometer. However, their MS/MS spectra (Post-Source Decay mass spectra) are very different from those for protonated peptides. For many peptides, the charge-localized derivatives form predominantly a-type fragment ions. Sidechain losses are also observed for some residues, and the mechanism for their formation may be similar to those through which the a-type fragments are formed. The goal of this project is to understand the mechanisms through which fragment ions are formed from charge-derivatized spectra, such that sequence information can be extracted from the PSD mass spectra.